This application claims priority to German application No. 199 22 351.3, filed May 14, 1999, herein incorporated by reference.
1. Field of the Invention
The invention relates to the use of water-insoluble polyoxyarylene-polysiloxane block copolymers for defoaming aqueous media.
In numerous industrial processes, aqueous formulations use surface-active substances in order, for example, to emulsify water-insoluble substances or to reduce the interfacial tension and so achieve better wetting characteristics. An unwanted accompanying effect of these surface-active substances, however, is that air introduced during the preparation and processing of such water-based formulations is stabilized as foam. Examples of processes in which such problems occur include papermaking, wastewater treatment, emulsion polymerization, and the manufacture and application of water-dilutable coating systems.
Foam of this kind, produced in the course of stirring or dispersing operations during manufacture or filling, prolongs production times and reduces the effective capacity of the production plant. Foam is disruptive to the application of coating materials since it leads to unwanted surface defects. As a result, the use of antifoams or defoamers in virtually all water-based systems is widespread and often indispensable.
In the past, a large number of formulations have been described which envisage the use of, for example, silicone oils, organomodified siloxanes, hydrophobic polyoxyalkylenes, mineral oils, natural oils and other hydrophobic liquids as defoaming substances. Frequently, the above-mentioned substances are used in combinations with hydrophobic solids, such as pyrogenic silicas, metal stearates or fatty acid amides, for example, which often intensify the foam inhibiting or defoaming effect.
In accordance with the present state of knowledge, it is critical to the defoaming action of a defoamer that it is able to penetrate into the foam lamellae and to weaken them until they collapse. To achieve this requires a controlled incompatibility (hydrophobicity) with the aqueous phase to be defoamed: if a defoamer is too compatible (hydrophilic), it will not be able to be very effective since it does not penetrate preferentially into the foam lamella. If the degree of incompatibility is too great, then defoaming is generally very good but there are frequently the unwanted side effects of surface defects, deterioration in wetting characteristics, and separation phenomena.
Consequently, the search for a suitable defoamer is always a search for the correct incompatibility/compatibility balance for the system to be defoamed, with the aim of getting as close as possible to the target hydrophobicity/hydrophilicity equilibrium.
2. Description of the Related Art
In the past, polyoxyalkylene-polysiloxane block copolymers have proven suitable for allowing this equilibrium to be established in a particularly variable manner, as is described, inter alia, in U.S. Pat. No. 3,763,021.
When polyoxyalkylene-polysiloxane block copolymers are used, advantage is taken of the fact that the polysiloxane blocks can be modified in a targeted manner with polyoxyalkylene units which, by virtue of their composition from hydrophilic and hydrophobic polyoxyalkylene units, can be adapted, in interplay with the polysiloxane block, to the above-mentioned desired hydrophobicity/hydrophilicity equilibrium.
The prior art, an example of which that may be mentioned here is DE-C-1 012 602, includes polyoxyalkylene-polysiloxane block copolymers in which the polyoxyalkylene block is defined as (CnH2nO)x. The index n is an integer from 2 to 4, it being essential that these polyoxyalkylene blocks always include xe2x80x94CH2xe2x80x94CHRxe2x80x94Oxe2x80x94 groups as a common structural element, R being a hydrogen atom, a methyl group or an ethyl group. Such polyoxyalkylene blocks are prepared by ionic addition polymerization of oxirane derivatives, such as ethylene oxide, propylene oxide and butylene oxide. Furthermore, it is also possible to use epoxides of longer-chain olefins as well.
Typically, however, polyoxyethylenes and polyoxy-propylenes are used in the polyoxyalkylene-polysiloxane block copolymers of the prior art.
By varying the oxyalkylene units, and in particular by modifying their respective proportions and their sequence in the polymer (random and/or in blocks), a large number of polyoxyalkylene-polysiloxane block copolymers are obtainable.
Because of the restriction to these basic units described above, however, in many cases it has not been possible to date to find the optimum incompatibility/compatibility balance required for certain defoamers.
It is therefore an object of the present invention to provide particularly suitable polyoxyalkylene-polysiloxane block copolymers for defoaming aqueous media, said copolymers making it possible to establish the above-described desired incompatibility/compatibility balance in a targeted manner better than as has been possible to date.
This object is surprisingly achieved by the use of water-insoluble polyoxyarylene-polysiloxane block copolymers for defoaming aqueous media of the general average formula I: 
where
R1 and/or R7 are R2 or xe2x80x94[R4]wxe2x80x94[R5]xxe2x80x94[R6]xe2x80x94R8,
R2 and R3 are identical or different alkyl radicals, preferably having 1 to 24 carbon atoms, or are unsubstituted or substituted aryl radicals, preferably having up to 24 carbon atoms, especially phenyl.
R4xe2x80x94is a divalent radical of the formula xe2x80x94Oxe2x80x94, xe2x80x94NHxe2x80x94, xe2x80x94NR2xe2x80x94, xe2x80x94Sxe2x80x94 or of the formula xe2x80x94[Oxe2x80x94Si(CH3)2]uxe2x80x94, where
u is from 2 to 200,
R5 is identical or different alkylene radicals, preferably having 1 to 24 carbon atoms, or is xe2x80x94CnH2nxe2x88x92fR2fxe2x80x94Oxe2x80x94CmH2mxe2x88x92gR2gxe2x80x94, where
f is from 0 to 12,
g is from 0 to 12,
n is from 1 to 18,
m is from 1 to 18,
R6 is identical or different radicals of the general formula xe2x80x94Oxe2x80x94C2H4xe2x88x92aR9aO)b(C2H4xe2x88x92aR10aO)cxe2x80x94, where
R9 is identical or different alkyl radicals having 1 to 4 carbon atoms and
R10 is a radical of the general formula II 
xe2x80x83but where, taking into account all radicals R6, the proportion of C2H4O groups among the total of all radicals R6 does not exceed 40% by weight, and
R11 can be identical or different and is a hydrogen, a hydrocarbon radical, preferably having 1 to 12 carbon atoms, or a halogen atom,
a is from 0 to 3,
b is from 0 to 100,
c is from 1 to 100, and
the sum (b+c) is from 3 to 200
xe2x80x83and the sequence of the individual polyoxyalkylene and/or polyoxyarylene segments xe2x80x94(C2H4xe2x88x92aR9aO)bxe2x80x94 and xe2x80x94(C2H4xe2x88x92aR10aO)cxe2x80x94 can be as desired and, in particular, embraces block copolymers, such as random polymers and combinations thereof,
R8 is a hydrogen, a substituted or unsubstituted alkyl radical, preferably having from 1 to 6 carbon atoms, or an acyl radical,
v is from 3 to 200,
w is 0 or 1,
x is 0 or 1,
y is from 0 to 200,
and, if y is 0, R1 and/or R7 are/is xe2x80x94[R4]wxe2x80x94[R5]xxe2x80x94[R6]xe2x80x94R8.
As evident from the description given above, the polyoxyarylene-polysiloxane block copolymers described in the context of this invention can also include polyoxyalkylene units in addition to the polyoxyarylene units.
In many cases, polyoxyarylene-polysiloxane block copolymers which have proven highly suitable are those obtained by means of combinations of two polyoxyarylene units with one or more polysiloxane copolymers. In that case, the distinction in the case of the polyoxyarylene copolymers can be a distinction in relation either to the relative proportion of the units used for the preparation of this copolymer or to the molecular weight of the whole polyoxyarylene unit, or else can lie in its functionality.
Different polysiloxane copolymers can be distinguished in accordance with both their molecular mass and their degree of branching or the number and/or relative position of the reactive groups used which are available for linkage.
The mode of action of the compounds of the invention is expressly not that they bring about particularly good compatibilization between constituents of a mixture that differs in polarity. Rather, it is the case that they themselves are just compatible enough not to induce any unwanted side effects such as surface defects, deteriorations in the wetting characteristics, or separation phenomena, without bringing about the compatibilization of other constituents.
An important factor in comparison with the prior art is that the compounds of the invention contain aryl radicals along the xe2x80x94CH2xe2x80x94CHRxe2x80x94Oxe2x80x94 chain (for example, R=phenyl) which also have a particularly positive effect on the incompatibility/compatibility balance which is essential for effective defoaming without unwanted side effects.
Such polyoxyarylene-polysiloxane block copolymers can be prepared in accordance with customary methods as in the prior art. As the skilled worker is well aware, such methods include in particular the hydrosilylation reaction of additionally olefinically unsaturated polyoxyarylene-polysiloxane block copolymers with hydropolysiloxanes or the nucleophilic substitution of polyoxyarylene-polysiloxane block copolymers with nucleophilic groups on polysiloxanes containing at least one silicon atom substituted by an electronegative group. Processes of this kind and the preparation of appropriate polysiloxanes are described, for example, in W. Noll xe2x80x9cChemie und Technologie der Siliconexe2x80x9d, Verlag Chemie, Weinheim, 1968.
In accordance with the prior art, the polyoxyarylene-polysiloxane block copolymers required for this purpose can be obtained by ionic polymerization of appropriately substituted oxirane derivatives.
The polyoxyarylene-polysiloxane block copolymers of the invention acquire their unique properties on the basis of the particular steric requirements of the polyoxyarylenes and of their capacity, markedly different from that of polyoxyalkylenes, to enter into interactions with one another and with other partners.
For the present invention it is essential that the polyoxyarylene-polysiloxane block copolymers of the invention are not water-soluble at the customary processing temperature of the media to be defoamed. This means that a 1:1 mixture (% by weight) with water leads to a nonhomogeneous mixture of the two phases and that no dissolving of the polyoxyarylene-polysiloxane block-copolymers of the invention takes place. However, small fractions of free polyoxyarylenes and/or polyoxyalkylenes may be present in the polyoxyarylene-polysiloxane block copolymers as a result of their preparation, as set out below. Since the preparation of these copolymers essentially obeys the laws of statistics, it is also possible for particularly oxyethylene-rich polyoxyarylenes and/or polyoxyalkylenes to occur, which in turn may be soluble in water. These fractions, however, are very small and in the polyoxyarylene-polysiloxane block copolymers of the invention are always less than 10% by weight.